Scrabble.cs

// example: dotnet run -- words_alpha.txt cat"

string dictionary = args[0];

string tiles = args[1];

Dictionary<string, List<string>> groups = File.ReadLines(path: dictionary)
    .GroupBy(Alphabetize)
    .ToDictionary(g => g.Key, g => g.ToList());

IEnumerable<List<string>> matches = PowerSet(tiles)
    .Select(Alphabetize)
    .Where(groups.ContainsKey)
    .OrderByDescending(s => s.Length)
    .Select(s => groups[s]);

foreach (List<string> group in matches)
    Console.WriteLine(string.Join(separator: ", ", values: group));

static IEnumerable<IEnumerable<T>> PowerSet<T>(IEnumerable<T> set) =>
    Enumerable.Range(0, (int)Math.Pow(2, set.Count()))
        .Select(b => set.Where((x, i) => ((1 << i) & b) != 0));

static string Alphabetize(IEnumerable<char> x) =>
    string.Concat(x.OrderBy(c => c));